SAN DIEGO (June 9, 2022) – The largest study ever conducted on a free-ranging population of rhinoceroses reveals that about one in every seven rhinos in a key South African national park has been infected with Mycobacterium bovis (M. bovis), the pathogen that causes bovine tuberculosis (bTB). The finding shines new light on the potential for diseases to disrupt global conservation efforts, and potentially increase risk to human health, if left unaddressed.
The study—conducted by Stellenbosch University (SU) Animal Tuberculosis Research Group, South African National Parks (SANParks) Veterinary Wildlife Services and San Diego Zoo Wildlife Alliance—tested samples from 437 rhinos, collected from 2016 to 2020 in Kruger National Park. Results revealed the presence of M. bovisinfection in an estimated 15.4% of the black and white rhinos studied.
While the research results prompted concern, evidence provided by the study is crucial to support effective conservation efforts to protect the already vulnerable rhino population. Added to this, scientists with the Animal TB Research Group, situated within SU’s Faculty of Medicine and Health Sciences, developed a novel diagnostic test to detect M. bovis infection in rhinos, which will greatly aid conservation efforts.
These results are also relevant in the U.S., where bovine TB is rare, but can occasionally be found in domestic cattle and other livestock. It can travel across landscapes by carrier species such as white-tailed deer, and it has reached endemic status among deer, bison and elk in parts of North America.
“There are examples of bovine tuberculosis in wildlife systems throughout the world,” said Carmel Witte, Ph.D., a quantitative epidemiologist with San Diego Zoo Wildlife Alliance and senior author of the study. “In the United States, spillover of infection from cattle has spread to white-tailed deer in Michigan. This, in turn, can create spillback of infection into livestock, and carries a risk of spread to other wildlife and people.”
Infected, but Asymptomatic
The researchers emphasized that the presence of infection does not mean that the animals are diseased or dying. Professor Michele Miller, who leads the Animal TB Research Group and is the National Research Foundation (NRF) South African Research Chair in Animal TB, said their research shows that most of the rhinos can contain the infection if they are otherwise healthy.
“It can be compared to humans who are infected with COVID-19 or have latent TB, but are asymptomatic,” Miller said. “The infected rhinos are harboring the bacteria, but their immune system is keeping it in check. They are not losing weight or coughing, and if you looked at a group of 400 rhinos, you wouldn’t be able to pick out those that are infected. They can potentially live for years with infection if it is contained.”
Witte added that the eventual population-level health effects of bTB are currently unknown. “Tuberculosis tends to be a disease that manifests over long periods of time, and when you compound an infectious disease with events such as climate change and unprecedented mortality due to poaching of endangered animals, it is cause for concern,” she said. “Continued surveillance of rhinoceros, as well as other animals, can help us understand the long-term impact of this infection in wildlife and prevent catastrophic population losses and further disease spread.”
Wynand Goosen, Ph.D., Wellcome International Training Fellow in the Animal TB Research Group, highlighted the importance of the further development of diagnostic tools and of a “Tuberculosis One Health” approach.
“Even though our research is very important from an animal conservation perspective, it is just as important from a human health-risk perspective,” Goosen said. “To avoid the next pandemic in people, livestock and wildlife will have to be actively monitored for various infectious pathogens with zoonotic potential. This requires appropriate diagnostic tools that are rapid and accurate. To develop these tools, research in all susceptible species is of the utmost importance.”
The study, which was published in the American scientific journal PNAS (Proceedings of the National Academy of Sciences) today (June 6, 2022), identified proximity to buffalo herds (for white rhinos) and a sampling year (for black rhinos) that coincided with periods of drought as risk factors for M. bovis infection.
A significant cluster of cases was detected near Kruger’s southwestern border, although infection was widely distributed across the rhinos’ range in the large national park. The identified cluster is close to the park’s border with the surrounding Mpumalanga province, consisting primarily of farmland with livestock herds that have historically been implicated in spillover of M. bovis to wildlife in Kruger, especially to buffalo.
Dr. Peter Buss, veterinary senior manager in Kruger National Park’s Veterinary Wildlife Services, added there is no evidence to suggest that bovine TB will have any impact on the population. “The rhinos are being exposed to the organism, they are mounting an immune response, but they are not getting sick and dying from it,” he said.“The same applies to other species. For example, we know that we get TB in our lions, and that individuals will die of the disease. But if you look at the population level of the disease, lions seem to be doing fine and their numbers have remained fairly static.”
The authors further emphasized that the findings were not a surprise, since bovine TB is prevalent in at least 15 other species in Kruger—one of Africa’s largest reserves—but that their research has significant positive implications for SANParks’ rhino conservation and management strategy.
“While this pathogen may not appear to drastically impact the health of rhinoceros individuals, it has significant implications for conservation management decisions,” explained Rebecca Dwyer, lead author of the study and a Ph.D. candidate in the Animal TB research team. “For example, tuberculosis testing in KNP rhinoceroses that are earmarked for translocation for conservation reasons can increase confidence of minimal risk of spread to other susceptible individuals at their destinations.”
“With South African rhinos being threatened by poaching, habitat loss and drought, it is key to be able to translocate them to strongholds where they can be kept safe, and to preserve their genetic diversity,” said Miller. “But bovine TB is a controlled veterinary disease, so once our research group, in partnership with SANParks, found bovine TB in Kruger rhinos in 2016, the Department of Agriculture, Land Reform and Rural Development (DALRRD) imposed movement restrictions to prevent spread of infection to other populations.”
These restrictions created a barrier to the movement of rhinos to other national or private reserves and has a significant impact on the conservation of the species, as Kruger has historically been an important source population of rhinoceros for other conservation strongholds in South Africa and other African countries.
The solution was to come up with a test to identify infected animals before they were moved to prevent disease transmission. According to Goosen, the screening test that was used in their study was validated by the Animal TB Research Group in 2019 and was recently approved by the DALRRD for use in Kruger rhinos (see info box below).
A management strategy involving a quarantine protocol and testing schedule was devised in collaboration with SANParks, and has been approved. “Should we now wish to start moving rhinos out of Kruger, we have that option to quarantine them and test them, and then send them out,” said Buss.
Dwyer added: “The findings of this study are significantly important for wildlife conservation—not just of rhinoceros, but of many other species in this context. It demonstrates that the spread of pathogens in multi-host systems has important consequences for conservation of different species and of the ecosystem as a whole.”
About San Diego Zoo Wildlife Alliance
San Diego Zoo Wildlife Alliance is a nonprofit international conservation leader, committed to inspiring a passion for nature and working toward a world where all life thrives. The Alliance empowers people from around the globe to support their mission to conserve wildlife through innovation and partnerships. San Diego Zoo Wildlife Alliance supports cutting-edge conservation and brings the stories of their work back to the San Diego Zoo and San Diego Zoo Safari Park—giving millions of guests, in person and virtually, the opportunity to experience conservation in action. The work of San Diego Zoo Wildlife Alliance extends from San Diego to eco-regional conservation “hubs” across the globe, where their expertise and assets—including the renowned Wildlife Biodiversity Bank—are able to effectively align with hundreds of regional partners to improve outcomes for wildlife in more coordinated efforts. By leveraging these skills in wildlife care and conservation science, and through collaboration with hundreds of partners, San Diego Zoo Wildlife Alliance has reintroduced more than 44 endangered species to native habitats. Each year, San Diego Zoo Wildlife Alliance’s work reaches over 1 billion people in 150 countries via news media, social media, their websites, educational resources and the San Diego Zoo Wildlife Explorers television programming, which is in children’s hospitals in 13 countries. Success is made possible by the support of members, donors and guests to the San Diego Zoo and San Diego Zoo Safari Park, who are Wildlife Allies committed to ensuring all life thrives.
More About the Diagnostic Test Used in the bTB Study
- It was developed based on a blood test used for TB detection in humans.
- It is an improvement on traditional diagnostic methods using culture because it is based on the immune response.
- The entire system is a modified commercial test which insures standardization.
- It is simple for in-field use and reproducible.
The Animal TB Research Group, over many years of research, has developed tests that can be used for African wildlife species. Improved tests are in demand by veterinarians throughout the world, since there are currently limited tests for diagnosing TB in wildlife. Tests have been developed for a variety of species, including African buffalo, warthogs, African elephants, African lions, cheetahs, leopards, wild dogs, hippos, baboons and meerkats.
Bovine TB in Other Kruger National Park Species
According to the researchers, the distribution of M. bovis infection in Kruger rhinoceros is similar to that reported for other species in the park. For example, a 1991–1992 survey of buffalo showed widespread bTB in the central and southern regions of the park, with individual herd prevalence of up to 67%. A later study (2012–2013) showed an overall infection prevalence of 44% in lions in the same areas. Such extensive infection is increasingly observed in species like warthogs, wild dogs and elephants, with cases identified in more than 15 species, to date. Taken together, these findings suggest that spillover of bTB is not a new occurrence and support the need for ongoing bTB surveillance across species to continuously assess disease risk and conservation impact.
Why Rhinos Are Translocated
Kruger National Park has been a source of rhinos for other locations that are starting new populations, and for maintaining genetic diversity—a strategy that has been adopted because rhinos were doing well in the park until the onset of poaching, explained Miller. “If a disaster affects an isolated population of an endangered species, that species could be lost. To avoid that risk, you don’t want all the animals in one location, and they are moved to multiple places. One of those risks is poaching. It is often easier to manage the security of rhinos in smaller reserves than in a huge, complex park. By moving animals to other locations, this ensures that there will be breeding populations, even if the threat of poaching continues in Kruger.”
Is bTB a Threat to Humans?
Although people can become infected with M. bovis, it usually only happens when they regularly handle infected (uncooked) animal organs or drink unpasteurized milk. Unlike diseases such as COVID-19, people need close prolonged contact to get TB and won’t contract it from visiting areas where infected animals may be present, Miller said.
Witte added that although most humans are not interacting with wild rhinos in a way that would put them at risk for acquiring M. bovis, the study highlights the ongoing potential for pathogen spill over from animals to people (and people to animals) at the human-domestic animal-wildlife interface.
According to Goosen, TB has similar consequences for humans and a broad range of animal species, yet cases in humans and animals are commonly treated as separate problems. A TB One Health approach is therefore warranted, by improving the surveillance of zoonotic mycobacteria in humans, livestock, wildlife and their environment throughout South Africa, using various novel technologies.
Ongoing Work in Kruger National Park
According to Buss, a lot of work still remains to be done on TB, especially the epidemiology, and particularly at a population level. “Although it has been around for maybe 60 to 70 years in Kruger, it would still be considered a relatively new disease. Because TB manifests itself so slowly, we would imagine that the disease is still expressing itself at a population level, and we still need to reach some sort of equilibrium with the disease—and we just don’t know exactly where that is.
“Although we might not see much happening at the moment, it is very difficult to predict the future,” Buss added. “It would really be helpful to have some idea of the potential impact of this disease, particularly in advance, should we be required to manage this. And that’s in all animals, not only rhinos.”
Prevalence in Other Parks and Reserves?
According to Buss, Kruger is currently the only national park where TB has been diagnosed in rhinos. “The problem is currently contained to the Kruger, thank goodness. But we would certainly want to keep it that way for other national parks.”
Follow this link to the full article in PNAS: https://www.pnas.org/doi/pdf/10.1073/pnas.2120656119
Proceedings of the National Academy of Sciences
Method of Research
Subject of Research
Epidemiology of Mycobacterium bovis infection in free-ranging rhinoceros in Kruger National Park, South Africa
Article Publication Date